1. Field of the Invention
This invention relates, in general, to compressing apparatus for compressing fluid, e.g., refrigerant, flowing through a refrigerating circuit. In particular, the invention relates to a rotary compressor for varying a compressing capacity thereof in response to load.
2. Description of the Related Art
A well known refrigerating circuit, e.g., an air conditioning circuit, is shown in FIG. 1. In FIG. 1, internal and external fan devices, accumulator, etc., which are conventionally used in a such circuit, are not shown for simplicity. A refrigerating circuit 11 includes a rotary compressor 13, a four-way valve 15, an external heat-exchanger 17, a decompressing device 19, e.g., capillary tube, and an internal heat-exchanger 21 connected in series. Refrigerating circuit 11 performs either a heating or cooling operation in response to the change in direction of fluid flow controlled by four-way valve 15. As is well known, rotary compressor 13 has a variable compressing ability responding to an air conditioning load. In such a rotary compressor, a driving unit 23 and a rotary compressing unit 25 are disposed in a hermetic casing 27. Rotary compressing unit 25 is driven by driving unit 23. The output frequency of an inverter circuit 29 is fed to driving unit 23, as a driving frequency. Thus, the compressing capacity of rotary compressing unit 25 can change in response to changes in the output frequency of inverter circuit 29 through driving unit 23, as indicated by a line A in FIG. 2.
However, in the above-described refrigerating circuit 11, rotary compressor 13, which changes a compressing capacity thereof in response to the output frequency of inverter circuit 29, has a lower limit compressing capacity corresponding to a prescribed output frequency, e.g., 30 Hz, of inverter circuit 29. This is because mechanical vibrations of rotary compressor 13 increase if the output frequency of inverter circuit 29 decreases below 30 Hz. Therefore, the compressing capacity of rotary compressor 13 can not be decreased beyond the lower limit compressing capacity thereof. Furthermore, decreasing the output frequency of inverter circuit 29 applied to rotary compressor 13 creates another problem. As shown in FIG. 2, the energy efficiency ratio (hereinafter referred as EER) of rotary compressor 13 is greatly reduced, as indicated by a curved line B, when the output frequency of inverter circuit 29 decreases. In particular, this phenomenon is caused by a decrease in the efficiency of driving unit 23 of compressor 13. Thus, the EER of rotary compressor 13 is not enhanced although the compressing capacity of rotary compressor 13 can be varied by inverter circuit 29.